The present invention relates to an electrostatic transfer type image formation apparatus that develops an electrostatic latent image on an image holder into a toner image by using a charged toner, and transfers this toner image onto an image-receiving unit.
In this type of an image formation apparatus, first, a latent image formation unit forms an electrostatic latent image corresponding to a draft image on an image section of the photo sensor as an image holder. Then, the developing unit develops the electrostatic latent image formed on the image section of the photo sensor. Consequently, a toner image is prepared using a charged toner on the image section of the photo sensor. A transfer unit transfers the toner image formed on the image section of the photo sensor onto a transfer material or an intermediate transfer unit like paper or an OHP sheet as the unit that receives the image.
In the intermediate-transfer type image formation apparatus that uses an intermediate transfer unit as the image-receiving unit, it is possible to form a color image as is known well. In forming a color image in this intermediate transfer type image formation apparatus, first, the latent image formation unit sequentially forms electrostatic latent images, that are a draft image resolved into four colors, onto a photo sensor as an image holder. Next, the developing unit sequentially develops the electrostatic latent images of the four colors formed on the photo sensor, thereby to sequentially form color toner images of four-color charged toners of yellow, magenta, cyan, and black, on the photo sensor. An intermediate transfer process is carried out four times to transfer the toner images of the four colors formed on the photo sensor onto the intermediate transfer unit, thereby to sequentially superimpose the four-color toner images on the intermediate transfer unit to complete a primary transfer. Thereafter, the four full-color toner images obtained by the primary transfer based on the superimposition on the intermediate transfer unit are collectively transferred onto a transfer material like paper or an OHP sheet to complete a secondary transfer. As a result, a full-color image is formed on the transfer material. Various proposals have been made for the intermediate transfer unit. These include the units that use a resin belt having a sufficient lubricating surface like polyimide, PVDF, and ETFE, and a rubber material like urethane, NBR, and CR.
Further, as another type of an image formation apparatus that forms a color image, there has been known a transfer drum type image formation apparatus that has the transfer material wound around a transfer drum that rotates in contact with the photo sensor in synchronism with this photo sensor. Based on this, toner images of various colors formed on the photo sensor are sequentially transferred onto the image-receiving material wound around the transfer drum. According to this transfer drum type image formation apparatus, the transfer material used for this image formation apparatus is limited to the one that can be wound around the transfer drum. Therefore, there is a limitation to the use of the transfer material, as compared with the transfer material that is used in the intermediate transfer type image formation apparatus. Further, the transfer pressure applied at the time of transferring the image changes depending on the thickness of the transfer material. Therefore, this has a disadvantage in that color registration becomes unstable.
In the above image formation apparatuses, a reduction in sizes of these apparatuses and an increase in the image formation speed have been demanded in recent years.
However, reducing sizes and increasing the image formation speed of the apparatuses in order to satisfy these requirements has had the following difficulty. It is not possible to sufficiently develop an electrostatic latent image that is formed on the image section of the photo sensor, according to the general developing system, as described later. Further, when the rate of adhesion of the charged toner to the electrostatic latent image (the developing efficiency) is increased to compensate for the shortage in the development of the electrostatic latent image, the quantity of toner adhered to a non-image section (the texture section, or a section where there is no image) increases. The toner must not adhere to this section in principle. Consequently, what is called a xe2x80x9ctexture stainxe2x80x9d phenomenon has occurred easily on the transfer image.
In order to sufficiently develop the image section of the electrostatic latent image formed on the photo sensor with the charged toner, it is usually necessary to form a development nip in the developing section between the photo sensor and the development roller, for example. This development nip has sizes that enable the securing of a developing time around 50 mm/sec to 100 mm/sec. Therefore, when the sizes of the apparatus are simply reduced or the image formation speed is increased, the sizes of the photo, sensor and the development roller are reduced, and it becomes impossible to form a development nip having sufficient sizes. Further, the rotation speeds of the photo sensor and the development roller are increased, which makes it impossible to secure a sufficient developing time.
Therefore, when the sizes of the apparatus are simply reduced or the image formation speed is simply increased in the image formation apparatus, it becomes impossible to secure a sufficient developing time, and the development efficiency of the electrostatic latent image is lowered at the developing time. In order to compensate for a reduction in the development efficiency due to the reduction in sizes and increase in the speed of the image formation apparatus, there has been the following method. This method is to increase the quantity of toner adhesion to the electrostatic latent image formed on the photo sensor by increasing the development bias. According to this method, the efficiency of developing the image section of the photo sensor improves. However, the quantity of the toner adhered to the non-image section of the photo sensor increases, and this generates the xe2x80x9ctexture stainxe2x80x9d on the transfer image.
Further, there is a method of using two development rollers which improve the development efficiency of the electrostatic latent image without changing the development bias. According to this method, however, it is necessary to prepare a new development roller, which leads to a cost increase. Further, installation space for this development roller is additionally required, which results in an increase in the sizes of the apparatus.
It is an object of the present invention to provide an image formation apparatus that can prevent a toner adhesion to a non-image section of an image holder and can form a satisfactory image with less stain on the texture.
The image formation apparatus according to the present invention comprises an image holder having a surface, a latent image formation unit that forms an electrostatic latent image on the surface of the image holder, a developing unit that develops the electrostatic latent image by using a charged toner, and an image-receiving unit to which a toner image on the image holder is to be transferred, a transferring unit that applies a transfer bias to the image-receiving unit to transfer the toner image onto the image-receiving unit. An amount of the transfer bias is set such that potential differences between surface potentials of an image section and a non-image section of the image holder and a surface potential of the image-receiving unit generate a discharging at the image section and do not generate a discharging at the non-image section.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.